The present invention relates to a negative pressure boosting device used as a brake booster or the like and, more particularly, to a negative pressure boosting device capable of exhibiting ideal input-output characteristics by providing different servo ratios for a low input range and for a high input range, respectively.
Conventionally, a negative pressure boosting device utilizing negative pressure is used as a brake booster in an automobile such as a passenger car. As an example of conventional typical negative pressure boosting devices, a negative pressure boosting device as shown in FIG. 4 is known. In FIG. 4, numeral 1 designates a negative pressure boosting device, 2 designates a front shell, 3 designates a rear shell, 4 designates a valve body, 5 designates a power piston comprising a power piston member 6 attached to the valve body 4 and a diaphragm 7 disposed between the shells 2 and 3, 8 designates a constant pressure chamber which is formed in a space between the shells 2 and 3 and is defined by the power piston 5 and into which negative pressure is always introduced, 9 designates a variable pressure chamber which is formed in the space between the shells 2 and 3 and is defined by the power piston 5 and into which atmospheric pressure is introduced during operation, 10 designates a valve plunger, 11 designates an input shaft connected to a brake pedal (not shown), 12 designates a valve element, 13 designates a first valve seat which is annular and is formed on the valve body 4, 14 designates a second valve seat which is annular and is formed on the valve plunger 10, 15 designates a vacuum valve composed of the valve element 12 and the first valve seat 13, 16 designates an atmospheric valve composed of the valve element 12 and the second valve seat 14, 17 designates a valve spring always biasing the valve element 12 in such a direction as to seat the valve element 12 onto the first valve seat 13, 18 designates an atmosphere inlet, 19 designates a vacuum passage, 20 designates a key member for restricting the movement of the valve plunger 10 relative to the valve body 4 to a predetermined value and defining the rear most positions of the valve body 4 and the valve plunger 10, 21 designates a spacer, 22 designates a reaction disk, 23 designates an output shaft, 24 designates a return spring, and 25 designates a vacuum pressure inlet. The vacuum valve 15 and the atmospheric valve 16 compose a valve mechanism of the conventional example.
In the negative pressure boosting device 1 having the aforementioned structure, negative pressure is always introduced into the constant pressure chamber 8 through the vacuum pressure inlet 25. In the inoperative state of the negative pressure boosting device 1, the valve body 4, the power piston 5, the valve plunger 6, the input shaft 11, and the output shaft 23 are positioned as shown in FIG. 4. In these positions, the vacuum valve 15 is open and the atmospheric valve 16 is closed. That is, the variable pressure chamber 9 is in communication with the constant pressure chamber 8 via the open vacuum valve 15 and the vacuum passage 19 and is isolated from the atmosphere. Therefore, negative pressure is introduced into the variable pressure chamber 9 so that there is no difference in pressure between the variable pressure chamber 9 and the constant pressure chamber 8.
Upon depression of the brake pedal for normal braking operation, the input shaft 11 is moved forward to move the valve plunger 10 forward. Accordingly, the valve element 12 is seated on the first valve seat 13 and the second valve seat 14 is spaced apart form the valve element 12 so as to close the vacuum valve 15 and open the atmospheric valve 16. That is, the variable pressure chamber 9 is isolated from the constant pressure chamber 8 and is in communication with the atmosphere. Therefore, the atmosphere is introduced into the variable pressure chamber 9 through the atmosphere inlet 18 and the open atmospheric valve 16. The result is a difference in pressure between the variable pressure chamber 9 and the constant pressure chamber 8. Because of the differential pressure, the power piston 5 is moved forward and the output shaft 23 is moved forward via the valve body 4 so as to move the piston of a master cylinder (not shown).
In the very initial stage of operation of the negative pressure boosting device 1, the forward movement of the valve plunger 10 moves the spacer 21, but the spacer 21 does not come in contact with the reaction disk 22 yet. Therefore, the reaction force is not transferred from the output shaft 23 to the brake pedal through the reaction disk 22, the spacer 21, the valve plunger 10, and the input shaft 11. As the input shaft 11 is further moved forward, the power piston 5 is also further moved forward so as to further move the piston of the master cylinder forward via the valve body 4 and the output shaft 23. Then, the valve plunger 10 and the spacer 21 are also further moved forward so that the spacer 21 comes in contact with the reaction disk 22. As a result, the reaction force is transferred from the output shaft 23 to the brake pedal through the reaction disk 22, the spacer 21, the valve plunger 10, and the input shaft 11. That is, the negative pressure boosting device 1 produces output corresponding to the input, thus exhibiting the jumping characteristic.
As the output of the negative pressure boosting device 1 reaches a predetermined value that is obtained by boosting the input of the input shaft 11 exerted with a pedaling force in accordance with a servo ratio, the atmospheric valve 16 and the vacuum valve 15 are both closed. In this state, the device becomes in the intermediate load state. Because of the output of the vacuum boosting device 1, the master cylinder produces braking pressure so that the wheel cylinders produce relatively large braking force i.e. boosted pedaling force, thereby operating the service braking.
As the brake pedal is released to cancel the braking operation from the state where the negative pressure boosting device 1 is in operation so that the atmospheric valve 16 and the vacuum valve 15 are both closed, the input shaft 11 and the valve plunger 10 are both moved backward. Therefore, the second valve seat 14 presses the valve element 12 backward so that the valve element 12 is spaced apart from the first valve seat 13 so as to open the vacuum valve 15. That is, the variable pressure chamber 9 is isolated from the atmosphere and is in communication with the constant pressure chamber 8. Accordingly, the atmosphere in the variable pressure chamber 9 is discharged to the constant pressure chamber 8 through the open vacuum valve 15 and the vacuum passage 19 and is then discharged further form the constant pressure chamber 8 to a vacuum source (not shown) through the vacuum inlet 25. As a result of this, by the spring force of the return spring 24, the valve body 4 and the power piston 5 are moved backward to the inoperative position shown in FIG. 4. According to the backward movement of the valve body 4, the output shaft 23 is also moved backward because of the spring force exerted by the return spring of the piston of the master cylinder, thereby canceling the service braking.
As the negative pressure boosting device 1 becomes in the full load range in the state that the pedaling force is great, i.e. the input of the negative pressure boosting device 1 is great, the boosting action according to the servo ratio of the negative pressure boosting device 1 is no more conducted so that the increase in output of the negative pressure boosting device 1 is proportional to the input of the negative pressure boosting device.
The aforementioned input-output characteristics of the conventional typical negative pressure boosting device 1 is shown in FIG. 2(d). After the jumping action is finished, the output is linearly increased according to the increase in input. Upon releasing the brake pedal when the input is in the intermediate load range of the negative pressure boosting device 1, the output is decreased along the same way as the increasing way according to the decrease in input. Upon releasing the brake pedal when the input is in the full load range over the intermediate load range, the output is decreased along a way of the full load range and is then decreased along a straight way to have such a hysteresis that the output is slightly greater than the output in the increasing way relative to the same input in the intermediate load range.
However, in FIG. 2(d), the negative pressure boosting device 1, as a conventional typical negative pressure boosting device, performs according to the input-output characteristic within a A-range when normal braking action is taken during the running of the vehicle in an urban area, while the negative pressure boosting device 1 performs according to the input-output characteristic within a B-range when braking action is taken with depression force stronger than that of the normal braking action, for example, for panic braking.
In recent years, it is required to ensure the controllability of brakes within the A-range corresponding to the normal braking action and it is required to make a driver to be aware of the stiff feeling (feeling as hard response from the brake pedal) within the B-range when the brake pedal is depressed with force stronger than that of the normal braking action.
As described concretely, as for the controllability of the brakes within the A-range, it is required that the output of the negative pressure boosting device is promptly varied (increased or decreased) corresponding to the variation (increase or decrease) in the input by braking action even with a little variation, thereby minimizing the hysteresis the input-output characteristic between in the input increasing way and the input decreasing way of the negative pressure boosting device 1.
On the other hand, in the B-range, for example, for panic braking with strong brake operating force, increased stiff feeling is required to make the driver to be aware of the strong braking operation. Of course, the output is required to be promptly decreased according to the releasing operation of the brake pedal. However, in this case, it is required to increase the hysteresis of input-output characteristic between the input increasing way and the input decreasing way of the negative pressure boosting device in order to prevent the output from being varied by a little increase in operational input.
However, in the conventional negative pressure boosting device 1, the input-output characteristics in the A-range and in the B-range are the same as shown in FIG. 2(d). That is, in the conventional negative pressure boosting device 1, the xe2x80x9cstiff feelingxe2x80x9d should be insufficient when the xe2x80x9ccontrollability of brakesxe2x80x9d takes priority, while the xe2x80x9ccontrollability of brakesxe2x80x9d should be poor when the xe2x80x9cstiff feelingxe2x80x9d takes priority. It is difficult to improve the xe2x80x9ccontrollability of brakesxe2x80x9d and to improve the xe2x80x9cstiff feelingxe2x80x9d at the same time.
It is an object of the present invention to provide a negative pressure boosting device in which the improvement of xe2x80x9ccontrollability of brakesxe2x80x9d in a low-input range and the improvement of xe2x80x9cstiff feelingxe2x80x9d in a high-input range are compatible.
To achieve the above-mentioned object, a negative pressure boosting device of the present invention comprises at least: a valve body which is disposed slidably relative to the inside of a space defined by a shell and air-tightly and slidably penetrates said shell; a power piston, connected to said valve body, for dividing said space into a constant pressure chamber into which negative pressure is introduced and a variable pressure chamber into which atmosphere is introduced during braking operation; a valve plunger which is slidably disposed in said valve body; an input shaft which connected to said valve plunger and is slidably disposed in said valve body; and a valve mechanism comprising a vacuum valve, which is disposed in said valve body and is controlled by the movement of said valve plunger to isolate or allow the communication between said constant pressure chamber and said variable pressure chamber, and an atmospheric valve which is disposed in said valve body and is controlled by the movement of said valve plunger to isolate or allow the communication between said variable pressure chamber and the atmosphere, and is characterized in that said valve mechanism is provided with a servo ratio changing means for changing the servo ratio during operation after input is initially exerted on said input shaft and before the input reaches the end of an intermediate load range.
Further, the present invention is characterized in that said servo ratio changing means sets the servo ratio to a ratio smaller than the servo ratio for the normal braking operation, or sets the servo ratio to a ratio smaller than the servo ratio for the normal braking operation and, after that, sets the servo ratio to the servo ratio for the normal braking operation again.
Furthermore, the present invention is characterized in that said vacuum valve is composed of a valve element which receives working pressure corresponding to the input exerted on said input shaft to move in the working direction, and a movable valve seat on which said valve element is seated and which is movable relative to the valve body, and said servo ratio changing means is composed of said movable valve seat and a control spring for controlling the start of movement of said movable valve seat relative to said valve body.
Moreover, the present invention is characterized in that said movable valve seat is always biased by the biasing force of said control spring toward the inoperative position and has a stopper which is engagable with said valve body, wherein the inoperative position of said movable valve seat is defined by that said stopper is engaged with said valve body by the biasing force of said control spring.
In addition, the present invention is characterized in that said movable valve seat is disposed slidably along the inner surface of a concavity formed in said valve body and that said control spring is compressed and disposed between said valve body and said movable valve seat.
In the negative pressure boosting device of the present invention having the aforementioned structure, the servo ratio can be changed during operation after input is initially exerted on the input shaft and before the input reaches the end of the intermediate load range of the negative pressure boosting device by the servo ratio changing means. Therefore, the servo ratio can be suitably set by the servo ratio changing means, thereby improving the controllability of the brakes in the low-input range, for example during service braking, and also giving the feeling as hard response, i.e. the stiff feeling to the driver in the high-input range, for example, during panic braking.
Since the servo ratio changing means is provided in the valve mechanism, the servo ratios can be advantageously and reliably changed by the servo ratio changing means.
Since the servo ratio changing means sets the servo ratio to a ratio smaller than the servo ratio for the normal braking operation, or sets the servo ratio to a ratio smaller than the servo ratio for the normal braking operation and then sets the servo ratio to the servo ratio for the normal braking operation again, thereby improving the controllability of brakes in the low-input range, while more properly giving the feeling as hard response, i.e. the stiff feeling to the driver, for example, during panic braking in the high-input range.
The servo ratio changing means is composed of a movable valve seat of a vacuum valve and a control spring for controlling the start of movement of the movable valve seat relative to a valve body, thereby simplifying the structure of the servo ratio changing means.
Further, a stopper is engaged with the valve body by the biasing force of a control spring so as to define the inoperative position of the movable valve seat, thereby easily controlling the inoperative position of the movable valve seat.
Furthermore, the movable valve seat is disposed slidably along the inner surface of the concavity formed in the valve body and the control spring is compressed and disposed between the valve body and the movable valve seat, thereby simplifying the structure of the valve mechanism even through it includes the servo ratio changing means.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.